1. Field of the Invention
The present invention relates generally to medical apparatus and methods, and more specifically to such apparatus and methods for dialyzing blood.
2. Description of the Prior Art
Various dialysis devices for single-needle dialysis are known. In single-needle dialysis, a single one-lumen cannula is inserted into a blood vessel of the patient. Connected to the cannula is a blood line, which branches into an arterial line and a venous line. A blood pump conveys the blood from the patient via the arterial line into an expansion chamber. In a second phase, the arterial line is closed off and the venous line opened. Now the blood is pumped out of the expansion chamber to a dialyzer and is thence returned to the patient via the open venous line. As a rule, two pumps are employed, one of which draws blood from the arterial line and the other which pumps blood into the venous line. Hose pumps are used, the delivery of which is proportional to the pump speed.
In a known dialysis apparatus of the above mentioned kind, the expansion chamber for the temporary uptake of the blood drawn through the arterial line is arranged between the arterial line and the pump. The pump outlet leads to the dialyzer. The dialyzer outlet is coupled to the venous line via a bubble trap. The arterial line and the venous line each contain a shutoff valve, both shutoff valves being pinched hose valves, and being operated alternately, so that one line is closed while the other is open. During the inflow phase the shutoff valve of the arterial line is open. The pump then draws blood through the arterial line into the expansion chamber, from which a corresponding volume of air is displaced. After the pressure in the venous line has risen to a certain value, the shutoff valve in the arterial line is closed and the shutoff valve in the venous line is opened. The blood present in the expansion chamber is pumped out by the blood pump which continues to run, and is supplied to the venous line via the dialyzer and the bubble trap. The switching between the inflow phase and the return flow phase depends on pressure measurements made at two different points of the closed blood circulation system.
The dialysis apparatus of the prior art entails certain disadvantages. The determination of pressure requires pressure lines and pressure measuring devices which are expensive, and susceptible to alteration with time that adversely affects the accuracy of the switching timing. Further the limit values of the pressure switching may be incorrectly adjusted. Another disadvantage resides in the fact that the permanently running pump always pumps into the venous line. When the venous line is shut off, the pressure in it increases to the limit value. When the limit value is reached, the shutoff valve in the venous line opens and the pressure discharges via the cannula into the blood vessel. But, at the same time, the pump continues to run, so that the normal pump delivery superimposes itself on the pressure discharge. This results in strong fluctuations in the quantity of blood supplied to the patient.